1. Field of the Invention
This invention relates generally to seismic prospecting and more particularly to apparatus for imparting variable frequency seismic energy to the earth.
2. Description of the Prior Art
Seismic vibrators normally comprise a vibratory prime mover, a coupling plate for coupling vibratory energy to the earth, a hold down mass for holding the coupling plate in contact with the earth during vibrations, and a reaction mass for generating a reaction impedance so that the vibratory energy produced by the prime mover will move the ground rather than the vibrator. Typically, the vibratory prime mover is a piston and cylinder mechanism, with the coupling plate attached to the piston and a reaction mass connected to the cylinder.
One of the main problems in seismic prospecting with a vibrator is the difficulty in generating a large reaction impedance. The usual way of generating a large reaction impedance is to use a large mass. However a large mass is unwieldy and hard to transport. The impedance of a mass also decreases with decreasing frequency making it particularly difficult to develop a large reaction impedance using only mass at low frequencies.